Ričardas Buividas

Mechanism of fine ripple formation on surfaces of (semi)transparent materials via a half-wavelength cavity feedback

The mechanism of the fine ripples, perpendicular to laser polarization, on the surface of (semi)transparent materials with period smaller than the vacuum wavelength, λ, of the incident radiation is proposed and experimentally validated. The sphere-to-plane transformation of nanoplasma bubbles responsible for the in-bulk ripples accounts for the fine ripples on the surface of dielectrics and semiconductors. The mechanism is demonstrated for 4H:SiC and sapphire surfaces using 800 nm/150 fs and 1030 nm/300 fs laser pulses. The ripples are pinned to the smallest possible standing wave cavity inside material of refractive index n. This defines the corresponding period, Λ = (λ/n)/2, of a light standing wave with intensity, E(2), at the maxima of which surface ablation occurs. The mechanism accounts for the fine ripples at the breakdown conditions. Comparison with ripples recorded on different materials and via other mechanisms using femtosecond pulses is presented and application potential is discussed.

Nano-groove and 3D fabrication by controlled avalanche using femtosecond laser pulses

We report fabrication of sub-100 nm resolution structures by ablation on the surface of sapphire using femtosecond laser pulses. A single 50–70 nm wide groove was recorded by laser ablation via a controlled ripple formation on the surface. Ripples are created by breakdown due to a sphere-to-plane formation of an ionisation below surface in a similar way as the bulk ripples. Different thresholds for the ripples formed parallel and perpendicular to direction of the laser scan were observed. In a sol-gel photo-polymer SZ2080 and thermo-polymer polydimethylsiloxane, free-standing 3D structures were formed without use of two-photon absorbing photo-sensitizers. Both cases of the surface and bulk structuring were achieved via a controlled avalanche, which dominated ionisation of materials.

Femtosecond laser induced density changes in GeO 2 and SiO 2 glasses: fictive temperature effect [Invited]

Density changes of GeO2 and SiO2 glasses subjected to irradiation by tightly focused femtosecond pulses are observed by Raman scattering. It is shown that densification caused by the void formation in GeO2 glass is very similar to the changes under hydrostatic pressure. In contrast, the experimental observations in SiO2 glass could be explained by pressure effect or by the fictive temperature anomaly, i. e., a resultant smaller specific volume of the glass (a denser phase) at a high thermal quenching rate. Density changes of GeO2 and SiO2 glasses are opposite upon close-to-equilibrium heating; this gives new insights into the mechanisms of densification under highly non-equilibrium conditions: fs-laser induced micro-explosions, heating and void formation. The pressure and temperature effects of glass modification by ultra-short laser pulses are discussed considering applications in optical memory, waveguiding, and formation of micro-optical elements.

Thermal imaging of a heat transport in regions structured by femtosecond laser

A non-contact determination of thermal diffusivity and spatial distribution of temperature on tens-of-micrometers scale is demonstrated by thermal imaging. Temperature localization and a heat flow have been in situ monitored with ∼ 10 ms temporal resolution in Kapton polymer films structured by femtosecond laser pulses. The structured regions can localize temperature and create strong thermal gradients of few degrees over tens-of-micrometers (∼ 0.1 K/μm). This is used to induce an anisotropy in a heat transport. Temperature changes on the order of ∼ 0.1°C were reliably detected and spatial spreading by diffusion was monitored using Fourier analysis. Application potential, miniaturization prospects, and emissivity changes induced by laser structuring of materials are discussed.

Plasmonic nano-printing: large-area nanoscale energy deposition for efficient surface texturing

The lossy nature of plasmonic wave due to absorption is shown to become an advantage for scaling-up a large area surface nanotexturing of transparent dielectrics and semiconductors by a self-organized sub-wavelength energy deposition leading to an ablation pattern—ripples—using this plasmonic nano-printing. Irreversible nanoscale modifications are delivered by surface plasmon polariton (SPP) using: (i) fast scan and (ii) cylindrical focusing of femtosecond laser pulses for a high patterning throughput. The mechanism of ripple formation on ZnS dielectric is experimentally proven to occur via surface wave at the substrate–plasma interface. The line focusing increase the ordering quality of ripples and facilitates fabrication over wafer-sized areas within a practical time span. Nanoprinting using SPP is expected to open new applications in photo-catalysis, tribology, and solar light harvesting via localized energy deposition rather scattering used in photonic and sensing applications based on re-scattering of SPP modes into far-field modes.

A bactericidal microfluidic device constructed using nano-textured black silicon

Nano-structured black silicon (bSi) was used as a substratum for the construction of a microfluidic device to test the bactericidal action of this nano-textured surface against Pseudomonas aeruginosa bacteria. A narrow 15 µm high and 1 cm wide flat flow channel was constructed that allowed the bacteria to come into contact with the bactericidal nano-spikes present on the surface of the bSi. The narrow channel within the device was designed such that a single layer of bacterial cells could reside at any given time above the bSi substratum during flow. The large 1 × 2 cm2 surface area of the bSi was shown to be efficient in being able to kill the bacterial cells, achieving an approximate 99% killing efficiency. The flow rate required to fill the bSi chamber was found to be 0.1 µL s−1, with a 10 min equilibration time being allowed for the bacterial cells to interact with the bSi surface. Complete rupturing of E. coli cells was achieved after 15 cycles, allowing the effective release of cellular proteins from within the bacterial cells (65.2 µg mL−1 from 3 × 108 cells per mL). The channel was then able to be re-used after washing of the cell with 10 successive cycles of sterile MilliQ water. Larger volumes of bacterial suspensions have the potential to be treated using a similar flow channel configuration if the dimensions of the flow channel are scaled accordingly. This bactericidal microfluidic device provides a novel platform for studies carried out under both static and dynamic (flow) conditions.

Photoluminescence from voids created by femtosecond-laser pulses inside cubic-BN.

Photoluminescence (PL) from femtosecond-laser-modified regions inside cubic-boron nitride (c-BN) was measured under UV and visible light excitation. Bright PL at the red spectral range was observed, with a typical excited state lifetime of ∼4  ns. Sharp emission lines are consistent with PL of intrinsic vibronic defects linked to the nitrogen vacancy formation (via Frenkel pair) observed earlier in high-energy electron-irradiated and ion-implanted c-BN. These, formerly known as the radiation centers, RC1, RC2, and RC3, have been identified at the locus of the voids formed by a single femtosecond-laser pulse. The method is promising to engineer color centers in c-BN for photonic applications.

Femtosecond laser drilling of optical fibers for sensing in microfluidic applications

We report on a technique for precise hole drilling in optical fibers using tightly focused femtosecond laser pulses. This direct laser writing approach makes it possible to minimize the amount of waveguide material for uncompromised mechanical performance of the fiber. The proof-of-the-principle of the fiber integration into a microfluidic chip is demonstrated. We show that fabricated holes in the waveguides can be used for measurement of absorption coefficient and refractive index changes at 1x10-3 refractive index units and 2 cm-1 for refractive index and absorption changes, respectively. Simple design and integration possibility of laser-fabricated waveguide sensors is prospective for optofluidic applications.

Hybrid subtractive-additive-welding microfabrication for lab-on-chip applications via single amplified femtosecond laser source

Abstract. An approach employing ultrafast laser hybrid subtractive-additive microfabrication, which combines ablation, three-dimensional nanolithography, and welding, is proposed for the realization of a lab-on-chip (LOC) device. A single amplified Yb:KGW femtosecond (fs)-pulsed laser source is shown to be suitable for fabricating microgrooves in glass slabs, polymerization of fine-meshes microfilter out of hybrid organic–inorganic photopolymer SZ2080 inside them, and, finally, sealing the whole chip with cover glass into a single monolithic piece. The created microfluidic device proved its particle sorting function by separating 1- and 10-μm polystyrene spheres in an aqueous mixture. All together, this proves that laser microfabrication based on a single amplified fs laser source is a flexible and versatile approach for the hybrid subtractive-additive manufacturing of functional mesoscale multimaterial LOC devices.

High-order harmonic generation from a dual-gas, multi-jet array with individual gas jet control.

We present a gas jet array for use in high-order harmonic generation experiments. Precise control of the pressure in each individual gas jet has allowed a thorough investigation into mechanisms contributing to the selective enhancement observed in the harmonic spectra produced by dual-gas, multi-jet arrays. Our results reveal that in our case, the dominant enhancement mechanism is the result of a compression of the harmonic-producing gas jet due to the presence of other gas jets in the array. The individual control of the gas jets in the array also provides a promising method for enhancing the harmonic yield by precise tailoring of the length and pressure gradient of the interaction region.